Diode lasers are used in many imaging applications as a convenient and low-cost radiation source. Material processing applications may make use of suitably coupled diode laser radiation to change the nature or character of a workpiece. Image recording and display systems may use laser diodes to provide illumination for an optical system.
In one particular imaging application, a monolithic array of laser diode emitters may be used to illuminate a multi-channel light valve. A light valve generally has a plurality of individually addressable modulator sites; each site producing a beam or channel of imagewise modulated light. An image is formed by selectively activating the channels while scanning them over an image receiver. For high quality imaging it is usually necessary that channels be uniform in their imaging characteristics, a requirement that presents a difficult challenge for system designers since the illumination from a laser diode is highly astigmatic with poor overall beam quality. Consequently optical systems for gathering and formatting the light output seek to overcome the inherent limitations of the diode laser output in order to produce useable illumination.
U.S. Pat. No. 5,517,359 to Gelbart describes a method for imaging the radiation from a laser diode array having multiple emitters onto a linear light valve. The optical system superimposes the radiation line from each emitter at the plane of the light valve, thus forming a single combined illumination line. The superimposition provides some immunity from emitter failures (either partial of full). In the event of such a failure, while the output power is reduced, the uniformity of the line is not severely impacted.
Even with superimposed emitters, the uniformity of the individual emitter radiation profiles still has an impact on the overall uniformity of the line. A good laser diode array can have emitters that are more than 20% non-uniform in the slow axis. When the radiation from a plurality of emitters is combined, some of the non-uniformities may offset each other but commonly 10–15% non-uniformity remains. Some light valves can accommodate this non-uniformity by balancing the output from each channel by attenuating output from channels that are more strongly illuminated. This however represents wastage of up to 15% of the useful light output since it is not possible to amplify weak channels.
U.S. Pat. No. 6,137,631 to Moulin describes a means for mixing the radiant energy from a plurality of emitters on a laser diode array. The mixing means comprises a plurality of reflecting surfaces placed at or downstream from a point where the laser radiation has been focused. The radiant energy entering the mixing means is subjected to multiple reflections, which makes the output distribution of the emerging radiant energy more uniform.
Laser diode arrays having nineteen or more 150 μm emitters are now available with total power output of around 50 W at a wavelength of 830 nm. While efforts are constantly underway to provide higher power, material and fabrication techniques still limit the power that can be achieved for any given configuration. In order to provide illumination lines with total power in the region of 100 W, an optical system designer may only be left with the option of combining the radiation from a plurality of laser diode arrays. Dual laser array combinations are disclosed in U.S. Pat. No. 5,900,981 to Oren et. al. and U.S. Pat. No. 6,064,528 to Simpson.